Endogenous substances such as adenosine, adenosine monophosphate and adenosine triphosphate interfere with release of sympathetic and parasympathetic neurotransmitters; secretion of catecholamines evoked by acetylcholine is not affected by adenosine compounds. Experiments are in progress to determine the mechanism by which these substances block release. Two possibilities are considered. One is that these agents could block Ca influx during excitation of nerve and thus limit the release. Another is that these agents could change electrical properties of the nerve membrane so that each stimulus is not as effective and thereby lowers Ca influx and release. Studies will be carried out in cultured nerve cells.